Continuing advances and changes in the types of fabrics, materials, and colors used in the apparel industry are putting increasing pressure on the garment and clothes cleaning industry. Brighter colors and more delicate fabrics require specialized care and tighter tolerances on every aspect of the cleaning cycle, including better regulation of water temperature. While businesses may be able to afford more sophisticated controls for their industrial washing machines to allow them to be better regulated, modern pressures on time and personal budgets diminish the desirability of new fashions that require professional cleaning only. Unfortunately, the current state of the art for water temperature control in consumer washing machines may not maintain the water temperature within tight enough tolerances for these new fabrics, materials, or colors.
In current washing machines the water temperature control regulates temperature of the water during the fill of the tub. This is done by having either the cold or the hot water valve on, and then modulating the other valve to maintain the aggregate temperature of the water in the tub within a certain range. The particular valve that is commanded to be continuously on (cold or hot) is selected via a user actuated water temperature control switch. If the user selects HOT on the temperature selector switch, for example, the controller may turn on the hot water valve continuously and modulate the cold water valve on and off to maintain the temperature at a pre-selected setpoint. If, on the other hand, a user selects WARM, the controller may turn on the cold water valve continuously and modulate the hot water valve to maintain the water temperature in the tub at a different pre-selected setpoint.
One problem with many conventional water temperature controllers which introduces an unacceptable tolerance error in the control set points may be better understood with reference to FIG. 5. As may be seen from this conventional temperature controller, a single reference voltage Vcc is used for both temperature set points as monitored by comparators 13 and 15. The actual temperature measurement is made by a thermistor 17 placed in the water flow. A pull-up resistor 19 biases the thermistor 17 to establish a voltage at node 11 used by comparator 13 to turn on a water solenoid driver circuit 21. A shunt resistor 23 coupled across the thermistor 17 is used to develop the second temperature reading for the comparator 15 to turn on a water solenoid driver circuit 21.
The particular water valve that is controlled by the solenoid driver circuitry 21 is determined by the position of selector switch 25. The particular comparator 13, 15 that is enabled for temperature sensing control is determined by comparators 27, 29 based on the position of the hot water valve (via input 31). When the hot water valve is turned on full time, comparator 15 modulates the cold water valve, and when the cold water valve is turned on full time, comparator 13 modulates the hot water valve. Unfortunately, this sensing of the hot water valve at input 31 and selection via comparators 27 and 29 increases the cost and complexity, and reduces the reliability of this circuit. Further, the requirement for the valve driver selector switch 25 and its associated driver circuitry 33 also adds to the cost and detracts from the reliability of the water temperature controller. In the highly competitive and cost conscience consumer appliance industry, the requirement for four comparators 13, 15, 27, and 29, a hot water valve sense input 31, and a selector switch 25 (and its associated circuitry 33) in the water temperature control circuit detrimentally impacts the marketability of appliances that incorporate such high cost circuitry.
There, therefore, exists a need in the art for a new and improved water temperature controller that has tighter tolerance on temperature variation, that is more cost effective, and that is less complex with a higher reliability than current designs.